Employed as the photodetector for PET detectors have been photomultiplier tubes (PMT). When incorporated into the PET device so as to be located on a surface facing to a subject (to be referred to as the top surface of the scintillator block), the PMT not only causes the occupied space thereof to invade the detection region but also serves as a scattering body during detection of radiation. On the other hand, coupling the PMT to a side surface of the scintillator block causes an increase in the region in which radiation cannot be detected, resulting in the PET device having a reduced sensitivity. Thus, as shown in FIG. 1(a), the PMT 12 was coupled only to the surface opposite to the subject (to be referred to as the bottom surface of the scintillator block 10). The two-dimensional position of a place at which radiation was absorbed in the scintillator block 10 is identified by coupling a plurality of PMTs or the position sensitive PMT (PS-PMT) 12 to the bottom surface and then performing the Anger-type calculation on signals therefrom, that is, performing a position calculation in the same manner as is performed to determine the center of gravity. Responses associated with the positions of absorption appear on the two-dimensional (2D) position histogram that represents the results of the Anger-type calculation. When a block of an array of small scintillator elements is employed in place of one large scintillator, the responses from each element appear discontinuously as illustrated in FIG. 1(b).
Under the condition that the PMT is coupled only to the bottom surface of the scintillator block, the scintillator block was improved in various ways so as to obtain a position in the direction of depth (DOI information) with respect to the photodetector. However, recently, semiconductor photodetectors such as avalanche photodiodes (APD) or Geiger mode APDs (also referred to as Si-PM or MPPC (Multi-Pixel Photon Counter) as a product name) have been rapidly developed, so that studies have been made on PET detectors with the PS-PMT replaced by the semiconductor photodetector. The semiconductor photodetector, which is small and thin, enables new detector designs, for example, such that a semiconductor photodetector having a reduced volume does not function as a scattering body even when the photodetector is disposed on the top surface of the detector. This fact has been utilized for studies on a DOI detection method (see Non-Patent Literatures 1 and 2) as shown in FIGS. 2(a) and (b), in which photodetectors are coupled to the top and bottom surfaces of an element array of the scintillator block 10 (a photodiode (PD) 14 on the top surface and the PS-PMT 12 on the bottom surface in FIG. 2(a), and a position sensitive APD 16 on both the top and bottom surfaces in FIG. 2(b)), so as to obtain DOI information by the ratio of signal pulse heights from the photodetectors on the top and bottom surfaces. The aforementioned fact has also been utilized for studies on a DOI detector in which the photodetector (APD 16) is coupled to a side surface as shown in FIG. 2(c) so as to identify the position in the DOI direction from the signal therefrom (see Non-Patent Literature 3). In the technique of FIG. 2(c) in which the photodetector is connected to a side surface, the detected position in the APD 16 is the DOI information itself, whereas the packing fraction is decreased in the PET device by the volume of the photodetectors though scintillation light can be more efficiently obtained with reduced loss of light because a wider surface of the scintillator elements is coupled to the photodetectors.
Furthermore, as shown in FIG. 2(d), the inventors have studied on the DOI detector in which semiconductor photodetectors 18 are disposed on the surface of the three-dimensional array 10A of small scintillator elements 10C in three dimensions (see Patent Literature 1 and Non-Patent Literature 4). In this structure, letting the three directions along the sides of the scintillator block be x, y, and z, photodetectors are disposed on each of the xy plane, the xz plane, and the yz plane, so that photodetector signals are operated to determine the x component, the y component, and the z component at a radiation absorption position. Other groups have suggested a detector structure in which the block is made up of one large scintillator having no optical discontinuity and photodetectors are disposed on each of the xy plane, the xz plane, and the yz plane. In one structure, a photodetector which is not of a position sensitive type is used to devise a method for identifying a radiation absorption position by simulation (see Non-Patent Literature 5), while another structure was used as an example of arrangements of photodetectors when a detector performance comparison was made by simulation between different arrangements of photodetectors (see Non-Patent Literature 6).